This invention relates, in general, to an electrophotographic photoreceptor and, in particular, a novel electrophotographic photoreceptor having a photosensitive layer containing an azo pigment. More specifically, it relates to an electrophotographic photoreceptor having a high sensitivity and a high durability suitable for repetitive use.
As for conventional electrophotographic photoreceptors, there have been widely known those having a photosensitive layer containing as principal constituent inorganic photoconductive materials such as selenium, zinc oxide, cadmium sulfide, and the like. Such photoreceptors, however, are not always satisfactory with respect to sensitivity, thermal stability, moisture resistance, or durability. Particularly, selenium and cadmium sulfide are produced or handled under restricted conditions because of their toxicity.
In recent years, electrophotographic photoreceptors having a photosensitive layer containing as principal constituent organic photoconductive compounds are attracting a wide interest on account of the advantages such as relatively easier production, lower cost, easier handling and superior thermal stability as compared with selenium photoreceptor. Poly-N-vinylcarbazole is a well known organic photoconductor. An electrophotographic photoreceptor having a sensitive layer containing as major constituent a charge transfer complex formed from poly-N-vinylcarbazole and a Lewis acid such as 2,4,7-trinitro-9-fluorenone is not satisfactory in sensitivity and durability.
A separated function photoreceptor of the superimposed type or dispersion type, in which the charge carrier generation function and the charge carrier transport function are assigned to different substances, has advantages in that the materials can be chosen from a large variety and it is possible to obtain comparatively easily a photoreceptor having desired properties with respect to charge characteristics, sensitivity, and durability.
Various charge carrier generation substances and charge carrier transport substances have been proposed. There is an electrophotographic photoreceptor in actual use, which has a combination of a charge carrier generation layer (CGL) comprising amorphous selenium and a charge carrier transport layer (CTL) comprising poly-N-vinylcarbazole as major constituent. The CGL comprising amorphous selenium, however, has a disadvantage of insufficient durability.
There are also proposed to use various organic dyes and pigments as charge carrier generation substance. For instance, electrophotographic photoreceptors which contain monoazo or bisazo pigments in the sensitive layer are disclosed in Japanese Patent Publication Nos. 30,513/73 and 11,945/81; Japanese Patent Application "Kokai" (Laid-open) Nos. 4,241/77 and 46,558/79. Such azo pigments, however, are not always satisfactory in sensitivity, residual potential, or stability in repetitive use. Moreover, their spectral wavelengths regions do not cover the near infrared region, leading to restricted freedom in the selection of light source. The fact is that there has been obtained no photoreceptor which sufficiently meet the broad range of requirements for electrophotographic processes.